BUENOS AIRES (Reuters) - A little known city administrator has emerged as a major player in Argentine politics after her surprise victory to lead the country’s biggest province and her role in the campaign of Mauricio Macri, the front-runner in Sunday’s presidential vote.

If Macri, the business-friendly mayor of Buenos Aires, beats the ruling party candidate four days from now, he will owe part of the victory to Maria Eugenia Vidal, who rose from obscurity over the last year to clinch one of the country’s top jobs.

She unseated the Peronist party from its traditional perch atop the Buenos Aires provincial government in the Oct. 25 general election. Macri defied the polls that same day by easily forcing a presidential run-off against ruling party candidate Daniel Scioli.

Vidal was nominated for Buenos Aires governor with the support of Macri back when she was a city official.

By winning the governorship of Peronist stronghold Buenos Aires, Vidal raised expectations that Macri may be able to do the same at a national level.

“Macri is going to do good things, just as we saw in the provincial election with Vidal. That was a sign of hope,” said Ana Montes, a nurse in the Buenos Aires suburb of La Matanza.

Scioli has been endorsed by President Cristina Fernandez, who is revered by the poor for widening the social safety net but reviled by investors for heaping controls on the economy.

Macri blames the ruling party for Argentina’s slow economy and promises to increase investment by a the economic imbalances left by years of Fernandez’s free-spending populism.

“With her middle-class roots Vidal acts as counterweight to Macri, whom Scioli’s Front for Victory party often portrays as a neo-liberal born into money and privilege,” said Jimena Blanco, an analyst with the Verisk Maplecroft consultancy in Britain.

Vidal, 42, even figures in one of Macri’s TV ads.

“How can you not be optimistic?” she says, her voice accompanying images of her and Macri talking with voters.

Macri promises a quicker shift toward free markets. He has a lead in the opinion polls but Scioli is still in the race.

“The greatest contribution that Vidal made to Macri is making people realize that change is possible,” said political analyst Alejandro Catterberg. “If Peronism was defeated in Buenos Aires province, it is easy to imagine it can be defeated on the national level.”

Vidal will start her four-year term as governor on Dec. 10, the same day that either Macri or Scioli will be sworn in as president.